Men who have sex with men (MSM) remain the group most affected by HIV infection in the United States with younger men at particular risk for new infection. Considerable attention has been devoted to developing intervention programs to prevent new infections among young MSM (YMSM); however, little research has been conducted on the social networks of YMSM and the social contexts where YMSM engage in high-risk behaviors that promote HIV risk, such as substance use (SU). Classical sociological theory and several prominent theories of health behavior, such as Social Cognitive Theory and the Theory of Reasoned Action, emphasize the importance of the social environment in influencing individual behavior. Yet little is known about the contexts in which YMSM gather to socialize and how these contexts facilitate engagement in SU and HIV risk behavior. The proposed study seeks to use a mixed-methods approach to examining social networks of YMSM in Los Angeles County and the contexts that are associated with varying levels of SU and HIV risk behavior among YMSM. Data from the NIDA-funded Healthy Young Men (HYM) study will be used to identify patterns in the social networks of YMSM in Los Angeles and the social contexts (e.g. bars, clubs, coffee shops, bookstores) where YMSM in Los Angeles gather. Using the Duality of Persons and Groups Theory, data on YMSM socialization patterns will be entered into matrices that can be analyzed as social networks (i.e., the shared contexts that YMSM nominate are regarded as ties between network members and network members who nominate a particular social context serve as ties between those contexts). Social networks will be analyzed with attention to sociodemographic characteristics (e.g., race/ethnicity, age, sexual identity). Attributes of individual YMSM, including SU behaviors and sexual risk behavior (SRB), will be associated with the social contexts that YMSM have nominated. During the second phase of the proposed study, qualitative methodologies (e.g., ethnographic observational data, semi-structured interviews) will be used to supplement quantitative findings in order to better understand the features of most popular social contexts among YMSM. Taken together, the sociometric network data and qualitative data will be used to inform the development of new or tailoring of existing interventions that are targeted to social contexts, which may be most conducive to the uptake of SU and HIV prevention messages among YMSM. PUBLIC HEALTH RELEVANCE: Young Men who have Sex with Men (YMSM) are disproportionately affected by HIV. This study will contribute to effective substance abuse and HIV risk behavior interventions for YMSM by understanding the social network and contextual influences that contribute to substance use and sexual risk in this population. Specifically, social network analysis coupled with qualitative fieldwork will elucidate characteristics of social networks and social contexts that are associated with varying levels of substance use and HIV risk behavior.